1. Field of the Invention
The present invention relates to an oscillator device that detects a temperature around a crystal unit and performs temperature compensation of an output frequency based on a temperature detection result.
2. Description of Related Art
In a case where a crystal oscillator is combined with applications requiring a significant high frequency stability, an oven controlled crystal oscillator (OCXO) is typically utilized. However, the OCXO has a large scale and a high power consumption. For this reason, a temperature controlled crystal oscillator (TCXO) having a simple structure and a low power consumption has been discussed in the art. However, the TCXO is disadvantageous in that its frequency stability against a temperature is lower than that of the OCXO.
FIG. 20 illustrates a typical structure of the TCXO. In FIG. 20, a crystal unit 90 and an oscillation circuit 91 are illustrated. The capacity of the voltage variable capacity element 92 is controlled by changing the control voltage supplied to a voltage variable capacity element 92 from the control voltage generator unit 93 to adjust the oscillation frequency (output frequency).
Since a frequency of the crystal unit 90 is changed depending on a temperature, the control voltage generator unit 93 corrects the control voltage depending on the temperature detected by the temperature detector 94. Specifically, a function obtained by normalizing a frequency temperature characteristic of the crystal unit 90 using a reference temperature, e.g., a third-order function is stored in a memory 95, and a frequency corresponding to a temperature detection value is read based on this function (frequency temperature characteristic). That is, how much a frequency at that temperature is deviated from the frequency at the reference temperature is read, and a control voltage corresponding to the frequency deviation as a temperature compensation amount is subtracted from the control voltage corresponding to the frequency at the reference temperature.
However, the data amount for specifying the function of the frequency temperature characteristic increases in order to perform accurate temperature correction control, so that a large-capacity memory 95 is necessary, which is expensive. In addition, since a thermistor is typically utilized as the temperature detector, it is difficult to anticipate improvement of the frequency accuracy due to a limitation in the detection accuracy of the temperature detector even when the aforementioned data amount increases.
In addition, the temperature detector 94 and the crystal unit 90 have a different arrangement position, thus it is difficult to obtain accurate temperature information of the crystal unit 90 in practice. At this point, it is also difficult to anticipate improvement of the frequency accuracy.
Referring to FIGS. 2 and 3 of Japanese Unexamined Patent Application No. 2001-292030, there is disclosed a configuration in which two crystal units (crystal resonators) are provided using two pairs of electrodes in a common crystal element. In addition, paragraph [0018] discloses that there is a frequency difference between two crystal units depending on a temperature change, so that it is possible to measure the temperature by measuring this frequency difference. In addition, a relationship between this frequency difference Δf and a frequency correction amount is stored in a read-only memory (ROM), and the frequency correction amount is read based on the frequency difference Δf.
However, in this technique, as disclosed in paragraph [0019], it is necessary to perform adjustment of the crystal units so as to establish a relationship f0≈f1≈f2 between a desired output frequency f0 and frequencies f1 and f2 of each of the two crystal units. Therefore, a manufacturing process of the crystal unit becomes complicated, so that it is difficult to obtain a high product yield. Furthermore, as disclosed in FIG. 4, clocks as a frequency signal from each crystal unit are counted for a predetermined time period, and a difference (f1−f2) therebetween is obtained. Therefore, the detection accuracy directly influences the detection time, so that it is difficult to perform temperature compensation with high accuracy.